(1) Field of the Invention
The present invention relates to an image signal processing device for correcting white balance by using a luminance signal and color-difference signals.
(2) Description of the Invention
Conventionally, the broadcast-use triniscope employing three separate cathode ray tubes for the red, green and blue images can keep a white balance of the image by covering itself with a color filter appropriate for a color temperature at the light source. When covering of the light source with the color filter does not correct the white balance of the image satisfactory, further white balance correction is provided by manipulating the RGB signal components with respective white balance coefficients. More specifically, the sum of the RGB signal components manipulated by respective white balance coefficients will be 1 if a white paper is shot. Thus, a slip in the white balance of the color image is found in its RGB signal components, and can be canceled by manipulating them with appropriate white balance coefficients. Such white balance correction is proved to be highly accurate.
This white balance correction method; however, can be applied to only the image signal processing device processing the RGB signals. On the other hand, the visual display unit having an image sensor, such as a CCD (Charge Coupled Device), generally processes the luminance signal and the color difference signals instead of the RGB signals, so that the above effective white balance correction cannot be applied thereto directly. Conventionally, the luminance and the color difference signals are converted into the RGB signals before the white balance correction is applied thereto. Formula 1 shows the white balance correction applied to the luminance and the color difference signals. In the formula, Y is the luminance signal; R-Y and B-Y are the color difference signals; and Kr, Kg, Kb are amplitude values of R signal, G signal, B signal respectively. ##EQU1##
A matrix locating in center of the right side converts the luminance and the color difference signals into the RGB signals; and a matrix locating at left end of the right side shows white balance coefficients to be applied to the RGB signal. As shown in Formula 2, Kr, Kg, Kb are related to each other so that the luminance level of the input color image signal will not change after the white balance correction, and Formula 3 will be obtained by substituting Formula 2 into Formula 1. ##EQU2##
The above white balance correction has the drawback that the circuit implementing this is very complicated and expensive; therefore, it is not suitable for a handy and simple image processing device such as a video use camera.
Conventionally, a video use camera corrects the white balance with a color control circuit which does not employ the white balance correction in Formula 3. FIG. 1 is a circuit block diagram showing the major part of the image signal processing device having such color control circuit. This image signal processing device comprises a color CCD image sensor 91, a color control circuit 92, a gamma correction circuit 93, and an encoder 94. The CCD image sensor 91 outputs the luminance signal Y and the color difference signals R-Y, B-Y responding to an input color image; the color control circuit 92 receives from the color CCD image sensor 91 the luminance signal Y, the color difference signals R-Y, B-Y, and applies white balance correction to these signals; the gamma correction circuit 93 applies gamma correction to the output of the color control circuit 92; and the encoder 94 converts the output of the gamma correction circuit 92 into NTSC (National Television Standard Committee) signal.
In this construction, the image signal processing device has the color control circuit 92 correct the white balance of the input image with reference to Formula 4. In the formula, Kr and Kb indicate white balance coefficients. ##EQU3##
According to the white balance correction in the formula, the color difference coordinates (R-Y, B-Y) of the input color image signal are transferred in parallel. In this correction, white color locating at origin can be corrected into the wanted color; however, other colors cannot be corrected into the wanted colors. That is, the further the coordinates of the input image signal are distant from the origin, in other words the image signal with higher level of color saturation, the less effective the white balance correction becomes. Results of such correction are shown in Table 1-1. In the table, ideal luminance signal and color difference signals are compared to the luminance and color difference signals obtained in Formula 4; and error is avoided by determining the white balance coefficients Kr and Kb to be -0.03 and 0.07 respectively for input of R=G=B=0.5. As apparent from the table, the corrected luminance and the corrected color difference signals do not agree with the ideal signals; especially B-Y signal for the input signals R=0, G=0.2, B=0 differs from the ideal signal by as high as 50%. Such difference in the signals will result in the corrected color image significantly different from the wanted color image.
Also the luminance signals and the color-difference signals in Table 1-1 are converted into the RGB signals to be expressed in 256 gradation. Table 1-2 shows the correction results expressed in 256 gradation. As shown in the table, it is apparent that the corrected RGB signals are significantly different from the ideal RGB signals. That is, the finding in Table 1-1 is confirmed in Table 1-2.